While solder is used in various apparatuses, recently, use of lead solder is restricted in some cases. On the other hand, the use of lead solder is not restricted in a field of aerospace. Since joint strength of a solder joint portion that uses such lead solder affects reliability of apparatuses, it is desired that the joint strength is high.
When a soldered portion connected by use of the above-mentioned solder is exposed to a high temperature environment, there is a problem that an intermetallic compound grows in the lead solder of the joint portion based on the Arrenius's rule, and metallic crystalline grains of the solder become coarse and consequently degradation of the joint strength progresses.
For example, Japanese Patent Application Laid-Open Publication No. 1993-259632 discloses a printed wiring board shown in FIG. 4.
As will be explained later, an object of the art is to improve reliability of joint in a joint process.
That is, according to the above-mentioned printed wiring board, a conductor 102, which forms a wiring pattern made of a copper foil and which is used for soldering electric components to be mounted, is arranged on both surfaces (surface A and surface B) of a substrate 100. A solder layer (first solder layer) 105, which includes lead and an intermetallic compound of tin, is arranged on the conductor 102 of the surface A (one surface). Moreover, a solder layer (second solder layer) 104, which is tin-lead based solder layer (tin-lead 63/37), is arranged on the conductor 102 of the surface B (the other surface). Here, a melting point (227 degrees centigrade) of the solder layer 105 on the surface A is higher than a melting point (183 degrees centigrade) of the solder layer 104 on the surface B.
As mentioned above, the solder layers, each of which has the same composition and is made of an alloy of tin and lead, are formed on the conductors formed on both surfaces of the substrate 102, and the solder layer formed on the one surface is changed to an alloy of lead and an intermetallic compound of tin by carrying out the heating process. Since the melting point of the solder layer formed on the one surface is higher than the melting point of the solder layer formed on the other surface by virtue of the above mention, it is possible to prevent that a joint portion of an electric component, which has been mounted on the surface A set downward, is melted when melting the solder layer 104 on the surface B.